WO 2006/099514 describes controlled release systems wherein an active principle and a bioactive polyelectrolyte complex (PEC) are added independently from each other to a polymer solution. They are incorporated in the polymer matrix in solution, by means of dissolution or dispersion. This document does not describe or suggest the specific use of acid-sensitive complexing agent, which interacts with the acid degradation products of the matrix. In WO 2006/099514, PECs, incorporated in polymer matrixes, modulate the release of a separate active principle from the PECs. These PECs are stable solids and can thus be considered to be bioactive. Furthermore, they cannot diffuse as such out of a polymer matrix of any kind. For this to be possible, the PEC must be destabilised and this is known to be very difficult. According to the invention, one of the partners of the complex is chosen to be degradable to this end.
Complexing may particularly mask excessive hydrophilicity and promote the incorporation of the active principle in the matrix. The incorporation of the complex in a matrix makes it possible to protect the complexed active principle temporarily, impede or prolong the delivery thereof and prevent a sudden initial release.